Garnett was born on May 19, 1976 in Greenville, South Carolina to Shirley Garnett. He was the second of his mother’s three children. Garnett’s mother never married his father, O’Lewis McCullough, with their relationship ending shortly after his birth. Garnett grew up with his mother and stepfather, Ernest Irby, with whom he did not get along, and two sisters.

Living with his sister in Chicago, Garnett led Farragut to a 28–2 record and was named National High School Player of the Year by USA Today. He was also named Mr. Basketball for the state of Illinois after averaging 25.2 points, 17.9 rebounds, 6.7 assists and 6.5 blocks while shooting 66.8% from the field. In four years of high school, Garnett posted an impressive 2,553 points, 1,809 rebounds and 737 blocked shots. In high school, Garnett played alongside Ronnie Fields, who also became a professional basketball player. Garnett was named the Most Outstanding Player at the McDonald’s All-American Game after registering 18 points, 11 rebounds, 4 assists and 3 blocked shots, and then declared himself eligible for the 1995 NBA draft. To mark the 35th anniversary of the McDonald’s All-American High School Boys Basketball Game, Garnett was honored as one of 35 Greatest McDonald’s All-Americans. Garnett’s decision not to play college basketball was influenced in part by his failure to score well enough on the ACT to meet NCAA requirements for freshman eligibility. Garnett told Student Sports Magazine in 1995 that if he had attended college he would have chosen to play college basketball for the University of Maryland.

Garnett was drafted with the fifth overall pick in the 1995 NBA draft by the Minnesota Timberwolves, and became the first player to be drafted directly out of high school since 1975. Since joining the NBA for the 1989–90 season, the Timberwolves had not won more than 29 games in any season. In Garnett’s rookie season, the Timberwolves were in the midst of a transition phase; they replaced Bill Blair with Flip Saunders as head coach early in the season, and made several trades. Garnett initially came off the bench in his rookie year, but moved into the starting lineup soon after Saunders became head coach and with the urging of Sam Mitchell. In the final 42 games of the year, averaged 14 points, 8.4 rebounds and 2.26 blocks as a starter. In his rookie year, Garnett and fellow newcomer Tom Gugliotta carried the scoring load. Garnett did not immediately leap to stardom as later prep-to-pro prospects such as Amar’e Stoudemire, LeBron James and Dwight Howard would, but he did have a very respectable rookie year. He was voted to the All-Rookie Second Team on averages of 10.4 points, 6.3 rebounds and 1.8 assists per game. Despite having some promising players, the Timberwolves suffered through their seventh consecutive sub-30 win season and failed to make the playoffs. At the time, Garnett was the youngest NBA player in history at 19 years and 11 months of age.

In August 1997, Garnett and Fleisher turned down the Timberwolves’ offer of a contract worth $102 million over six years. They thought there would be more offered to them on the basis of the signings of $105 million over seven years for Alonzo Mourning of the Miami Heat and $100.8 million over seven years for Juwan Howard with the Washington Bullets. To get out of the spotlight while negotiations were ongoing, Garnett stayed in Fleisher’s Westchester County home, north of New York City. One hour before the deadline on 1 October 1997, the Timberwolves and Garnett agreed on a six-year contract extension that was worth an unparalleled $126 million. The contract was considered a risky move and many analysts speculated that the deal would make it impossible for the Wolves to sign new players or even keep their own. The enormous size of Garnett’s contract was considered, by numerous sports writers, a major cause of labor tensions between players and owners that led to a lockout which shortened the 1998–99 NBA season. Despite the furor over his new contract, Garnett continued to improve, averaging 18.5 points, 9.6 rebounds, 4.2 assists, 1.8 blocks, and 1.7 steals per game. Again, he was an All-Star, and the Timberwolves finished with their first winning record in franchise history (45–37 for the season). For the second consecutive year, the young Timberwolves bowed out of the playoffs in the first round, this time losing 3–2 to the Seattle SuperSonics and superstar point guard Gary Payton. The two wins against the Sonics marked the Wolves’ first-ever playoff game wins. The off-season started poorly for the Timberwolves though as 20-point per game scorer Tom Gugliotta left for the Phoenix Suns.

His nicknames include “The Big Ticket”, “KG”, “The Kid” and formerly “The Franchise” (after being known as the Minnesota Timberwolves’ franchise player). He mentally prepared himself before games by banging his head against a padded basketball stanchion. Although Garnett is officially listed as 6 ft 11 in (2.11 m) by the NBA, he is widely accepted to be at least 7 ft 0 in (2.13 m) tall. During the 2007 NBA All-Star Game, Garnett admitted in an interview with Craig Sager to be “6 ft 11 in and some quarters” tall. Measurements from the 1995 NBA draft indicated that Garnett, who was 19 years old at the time, was 6 ft 11 in (2.11 m) tall barefoot. In 1997, Garnett was measured by the Minnesota Timberwolves training staff to be 7 ft 1 in (2.16 m) tall in his basketball shoes. In December 2011, Garnett agreed to become a limited shareholder of American-owned Italian Serie A football team A.S. Roma.

On May 20, 2000, Timberwolves’ guard and Garnett’s close friend Malik Sealy was killed by a drunk driver shortly after celebrating Garnett’s 24th birthday. Later that year, the NBA ruled that the free agent signing of Joe Smith was illegal. The league punished the team for the illegal signing by stripping them of three first-round draft picks, fining Glen Taylor (the owner of the team) $3.5 million, and banning general manager Kevin McHale for one year. In the 2000–01 NBA season, Garnett led the Wolves to a 47–35 record and made the All-NBA Second Team, but again, the Wolves did not survive the first round of the playoffs, losing to the Spurs 3–1.

In the 2000 Summer Olympics, Garnett won a gold medal as a member of the United States national team. In his first and only FIBA tournament, Garnett averaged 10.8 points, 9.1 rebounds and 2.1 assists per game.

In July 2004, Garnett married his longtime girlfriend Brandi Padilla during a private ceremony in California. Due to the wedding, Garnett did not take part in the Athens Olympic Games. The couple has two daughters. On July 12, 2018, Garnett’s wife filed for divorce, asking for custody of the children. Garnett is the half-brother of former basketball player Louis McCullough. Another professional basketball player, former Los Angeles Laker Shammond Williams, is his cousin.

During the 2007 off-season, Glen Taylor admitted that although he had planned on retaining Garnett, he would finally listen to trade offers. Garnett’s name was mentioned in various trade rumors involving the Chicago Bulls, Los Angeles Lakers, Golden State Warriors, Indiana Pacers, Boston Celtics, Phoenix Suns and Dallas Mavericks.

On July 31, 2007, Garnett was traded to the Boston Celtics in exchange for Al Jefferson, Ryan Gomes, Sebastian Telfair, Gerald Green, Theo Ratliff, cash considerations, Boston’s 2009 first-round draft pick (top 3 protected), and the 2009 first-round pick which Minnesota had traded to Boston in the Ricky Davis–Wally Szczerbiak trade of 2006. The 7-for-1 deal constitutes the largest number of players traded for a single player in league history. At the time of the trade, Garnett had the longest current tenure of any player in the NBA with one team, having played for the Timberwolves for his first 12 seasons (a total of 927 games). Garnett said that he was proud to be a part of the Celtics, and hoped to continue its proud tradition and basketball success. On the day the trade was announced, Garnett signed a three-year, $60 million contract extension that would start after his prior deal ran out in 2009. On August 1, the day after signing with the Celtics, Garnett threw the ceremonial first pitch at Fenway Park prior to a Red Sox–Orioles game.

In the 2008–09 season, Garnett started all of the 57 games he was able to suit up for. He averaged 15.8 points, 8.5 rebounds and 2.5 assists. On October 31, 2008, Garnett became the youngest player in NBA history to reach 1,000 career games, at 32 years and 165 days. Garnett earned his twelfth consecutive All-Star Game start on February 15, 2009. Following the All-Star Game, during a game against the Utah Jazz, Garnett strained his right knee late in the second quarter. The injury occurred on February 19, 2009, while attempting to go up for an alley-oop. He was forced to miss the next 14 games. Upon his return from the injury, he averaged 9 points and 4.5 rebounds in four games before being shut down for the season permanently, missing the final 25 games of the regular season, as well as the 2009 playoffs due to a right knee sprain. Without Garnett, the Celtics advanced to the Eastern Conference semi-finals, but were eliminated by the Orlando Magic.

In the 2009–10 season, Garnett and the Celtics, joined by newly signed free agent Rasheed Wallace, struggled with injuries and inconsistency throughout much of the regular season and earned the fourth seed in the Eastern Conference playoffs. Garnett was selected to play in the 2010 NBA All-Star Game (his 13th All-Star Game selection). Despite being written off by nearly every major sports analyst, the Celtics elevated their play and consistency, and dominated opponents much as they did during their 2008 championship run. They eliminated the Miami Heat, Cleveland Cavaliers, and Orlando Magic to advance to face the Los Angeles Lakers in the 2010 NBA Finals. In Game 6 against the Cavaliers, Garnett recorded 22 points, 12 rebounds and 3 assists in the series-clinching win. The 2010 Finals went to a decisive seventh game in Los Angeles, where the Celtics led well into the third quarter before the Lakers mounted a comeback and held on for the victory.

In the 2010–11 NBA season, Garnett and the Celtics started strong, winning 23 of their first 26 games. On December 30, 2010, Garnett injured his right knee after he tried to dunk. He missed two weeks with the injury. Garnett returned on January 17, 2011 to face the Orlando Magic. The Celtics ended the regular season third in the Eastern Conference behind the Chicago Bulls and the Miami Heat. Garnett averaged under 15 points, under 9 rebounds, and a career low 0.8 blocks per game. After sweeping the New York Knicks in the first round, they faced the Heat in the semi-finals. After losing the first two games of the series, Garnett’s playoff high 28 points helped the Celtics take Game 3. However, the Heat won the next two games, winning the series 4–1.

On June 30, 2012, Garnett agreed to a three-year contract extension with the Celtics worth an estimated $34 million. On January 17, 2013, it was announced that Garnett had been voted to start in the 2013 All-Star Game in Houston. On February 7, 2013, Garnett recorded his 25,000th point in a 116–95 victory over the Los Angeles Lakers.

On June 27, 2013, the day of the NBA draft, the Boston Celtics and the Brooklyn Nets reached a deal to trade Garnett, Paul Pierce and Jason Terry for future first-round picks in the 2014, 2016, and 2018 drafts, as well as Kris Humphries, Gerald Wallace, Kris Joseph, MarShon Brooks and Keith Bogans. The deal was finally completed on July 12, with Brooklyn also receiving D. J. White. Garnett chose to wear number 2 to honor his former Minnesota Timberwolves teammate Malik Sealy.

On December 13, 2013, Garnett surpassed 14,000 career rebounds, becoming only the tenth player in NBA history to do so. In reaching the milestone, Garnett also joined Kareem Abdul-Jabbar and Karl Malone as the only players to reach 25,000 points, 14,000 rebounds and 5,000 assists. He reached the milestone in the third quarter of a 103–99 road loss to the Detroit Pistons. The Nets cherished Garnett’s leadership and passion. Despite boasting a starting line-up of Deron Williams, Joe Johnson, Brook Lopez, Paul Pierce, and Garnett, the Nets were unable to advance past the second round of the playoffs, while Garnett finished his 19th NBA season with career low averages of 6.5 points and 6.6 rebounds per game.

On November 1, 2014, Garnett had arguably his best game for the Nets as he recorded 18 points and 14 rebounds in 35 minutes of action in the Nets’ 102–90 win over the Detroit Pistons. Six days later, he recorded five rebounds in a 110–99 win over the New York Knicks and by doing so, passed Walt Bellamy for ninth place on the all-time rebounding list.

On February 19, 2015, Garnett agreed to waive his no-trade clause in order to be traded back to Minnesota in exchange for Thaddeus Young. Six days later, he made his return for the Timberwolves against the Washington Wizards at the Target Center, recording five points on 2-of-7 shooting with eight rebounds and two blocks in 19 minutes in his first game for Minnesota since 2007. He appeared in just five games for the Timberwolves in 2014–15, before sitting out the team’s final 21 games of the season due to a nagging knee injury.

On July 11, 2015, Garnett re-signed with the Timberwolves to a two-year deal. On November 15, 2015, against the Memphis Grizzlies, Garnett became the fifth player in NBA history to play at least 50,000 minutes, joining Kareem Abdul-Jabbar, Karl Malone, Jason Kidd and Elvin Hayes. On December 1, against the Orlando Magic, Garnett surpassed Kidd (50,110) for third on the NBA’s career minutes list. Four days later, in a loss to the Portland Trail Blazers, he became the 15th player in NBA history to surpass 26,000 career points. On December 11, Garnett passed Malone as the NBA’s all-time leader in defensive rebounds during Minnesota’s 111–108 overtime loss to the Denver Nuggets. He finished the game with four defensive boards to give him 11,409 for his career, three ahead of Malone. Garnett appeared in 38 of the team’s first 45 games of the season before missing the entire second half of the season with a right knee injury, the same knee that kept him out of 25 games in 2008–09 when he was playing for Boston, as well as much of his post-trade time in Minnesota during the 2014–15 season.

On September 23, 2016, Garnett announced his retirement after 21 seasons in the NBA. While Garnett did express interest in playing for one more season with the Timberwolves, primarily with the goal in helping the team make it to the playoffs again with its promising young players and new head coach, he also told the team’s owner that he wasn’t sure that his knees would hold up for one more season.

In October 2016, Garnett joined the crew on TNT’s Inside the NBA. The following month, he became the host of his own segment, Area 21. In January 2017, Garnett joined the Los Angeles Clippers as a consultant. He also consulted with the Milwaukee Bucks during the 2016–17 season.

Garnett plays a fictionalized version of himself in the 2019 crime thriller film Uncut Gems, directed by the Safdie brothers. The movie is set in 2012 and revolves around the Eastern Conference Semi-finals series from that year between the Boston Celtics and Philadelphia 76ers. Garnett makes a trip from Philadelphia to a jewelry store owned by Adam Sandler’s character Howard Ratner in New York’s Diamond District and takes a liking to an Ethiopian opal which Ratner shows him; this leads to events that eventually spiral into chaos as the movie goes on.